The invention relates tho a surgical instrument for mechanically removing bone cement according to the precharacterizing part of claim 1.
Such surgical instruments are required for orthopaedic operations, e. g. for removing the remaining bone cement, e.g. of PMMA, after removal of an endoprosthesis (hip, knee, shoulder), and to allow perfect fit of the endoprosthesis to be newly set in.
For removing the cement e.g. chiselling tools are driven into the cement with the aid of a hammer to separate the cement from the bone in the medullary cavity.
Removal of the bone cement is preferably required in the field of orthopaedic surgery when loose or infected endoprostheses are to be replaced or removed.
The number of prosthesis replacements is continuously growing, in particular in the field of hip endoprosthetics. Removal of the bone cement used for prostheses fixed with cement is laborious and time-consuming. Hitherto the cement has normally been removed by means of chisels of different shapes, the safe employment of which is problematic e. g. due to poor visibility in the depth of the medullary cavity. Besides the high expenditure of time damage to the bone may occur, which would render new implantation of a prosthesis impossible or result in excessive loss of bone.
Extraction instruments are employed either without direct visual check or the position must be checked by means of X-ray image intensifiers. Malpositioning is possible and cannot be completely precluded even when X-ray image intensifiers are used for checking purposes. Malpositioning leads to damage of the bone which necessitates extended operations.
The employment of X-ray image intensifiers involves the risk of radiation exposure of the patient and the operation personnel. Known extraction instruments, in particular drills, lead, due to their configuration, to a forced pressure increase in the medullary cavity during penetration of the cement or the medullary block in peripheral direction, which may cause fat embolisation.
In the event of a hip joint replacement operation the cement used to fix the old prosthesis must be removed prior to setting in the new prosthesis. Since the thigh bone is a long tubular bone, the distal cement residue and the medullary block are difficult to access via the opening at the proximal end.
During the removal of the cement and/or the medullary block conventional tools produce a pressure increase in the bone below the medullary block. When work is carried out on the distal cement residue and the medullary block, respectively, a force is applied thereto. This may lead to compression of the space behindxe2x80x94an effect similar to that caused by an air pumpxe2x80x94and result in a pressure increase due to which the abundant fat in the medullary space can be pressed into the blood stream. This leads to fat embolisms which frequently result in permanent injury of the patient and in many cases even to death. Investigations have shown that a fat embolism is possibly not covered by anaesthesia. Death occurs suddenly and unforeseen. Fat embolisms are particularly critical in the case of a sepsis since germs are pressed into the blood stream together with the fat such that the sepsis is transmitted.
It is an object of the invention to provide a surgical instrument for removing distal cement residues and medullary blocks, which can prevent malpositioning of the extraction tool and pressure increases in the medullary space.
According to the invention the drilling tool and the extraction tool preferably comprise a bore extending coaxially to the respective tool shaft for receiving a centering bolt, the bore diameter of the coaxial bores of the drilling tool and the extraction tool are substantially adapted to the diameter of the centering bolt, and the centering bolt anchored at a predetermined location in the cement or the medullary block and in the medullary space located distally below is configured as an axial guide for the drilling tool and the extraction tool, respectively.
An essential feature of said extraction tool is that at least the distal end of the extraction tool is configured as a cannula in its central portion. When cement is removed, a centering bolt is first inserted into the distal cement residue and/or the meduallary block. By means of a drill which is of cannular configuration at least at the distal end the cement residue and/or the medullary block is completely pierced, wherein the centering bolt is configured as an axial guide. After predrilling the extraction tool is inserted into the bore previously produced down to the distal end of the cement residue and/or the medullary block, wherein the remaining portion of the centering bolt is again configured as a guide. Then the operator pulls out the distal cement residue and/or medullary block. If the cement residue is firmly embedded in the bone cement and/or the medullary block, the operator may beat with a special hammer on a shoulder surface of the extraction tool thus knocking out the extraction tool together with the cement block or the medullary block.
The cannular configuration of the extraction tool presents the following advantages:
Since the centering bolt serves as a guide, exact positioning of the instrument point is possible despite difficult access in the long tubular bone and poor visibility. This prevents perforation or fissured fracture of the bone due to excursion of the extraction tool during predrilling of the bone and insertion of the extraction tool. Perforation or fissured fracture of the bone results in a slower healing process and may, in the worst case, render implantation of the new prosthesis impossible.
The coaxial bores are open at the proximal end thus forming a pressure compensation duct.
When using the drilling tool of cannular configuration and the extraction tool of cannular configuration, a pressure compensation via the axial bore takes place prior to and during removal of the cement residue and/or the medullary block, the pressure compensation preventing a pressure increase in the medullary space below the cement residue and/or the medullary block such that the risk of fat embolism or transmitted sepsis is reduced.
The extraction tool preferably comprises a self-cutting male thread which can be inserted at a low expenditure of force into the bore produced by the drilling tool.
The male thread is preferably configured as a cortex screw such that the male thread is anchored in the predrilled bore due to a self-locking effect. The coaxial bores provided in the extraction tool and the drilling tool for receiving the centering bolt have a diameter ranging from approximately 1.5 to 4 mm, preferably approximately 2-3 mm.
The centering bolt has a smaller diameter with respect to the coaxial bores of the drilling tool and the extraction tool. This allows both the drilling tool and the extraction tool to be guided, with a clearance, by the same centering bolt such that exact positioning of the tools is possible.
The distal ends of the coaxial bores of the drilling tool and the extraction tool may be flared towards the outside. This flared configuration of the distal end of the bores facilitates insertion of the tools.
The coaxial bores provided in the drilling tool and/or the extraction tool may comprise a stop defining the maximum penetration depth of the tool. In this manner the tools can be prevented from being too deeply inserted into the distal medullary space and/or the cement residue or the medullary block.